The equilibrium between the oxidation of hydrogen peroxide by oxygen and the dismutation of peroxyl or superoxide radicals in aqueous solutions in contact with oxygen

Abstract

The equilibrium O₂ + HO₂⁻ + OH⁻ = 2O₂⁻ + H₂O, which plays an important role in the chemistry of H₂O₂ in aqueous media, including H₂O₂ bleaching of wood pulps and other materials, has been overlooked in the literature. Usually only the backward reaction is considered. We have calculated the equilibrium constant of this reaction from literature data on reversible electrochemical cell measurements without a liquid junction. Re-analysis of these data reveals the reason for a long-standing discrepancy between experiments and predictions based on thermal and equilibria data of the National Bureau of Standards: since HO₂⁻ is in equilibrium with O₂⁻, the actual HO₂⁻ concentration is less than that assumed. We find that the standard redox potential of the oxygen/superoxide couple is −0.137 V, different from the widely quoted literature value of −0.33 V in irradiated solutions. Re-evaluation of pulse radiolysis equilibria on duroquinone-superoxide yields a redox potential for the O_2(g)/O_2(aq)⁻ couple of ca. −0.13 V. The standard Gibbs energy of formation of the superoxide radical is found to be 13.18 kJ mol⁻¹ (3.15 kcal mol⁻¹) and that of the peroxyl radical −14.69 kJ mol⁻¹ (−3.51 kcal mol⁻¹).